U.S. Pat. No. 3,542,737 discloses poly(ethylene terephthalate) polymers containing up to 15 mole percent of a substituted succinic anhydride, the polymers being useful to produce high melting films and fibers. U.S. Pat. No. 3,890,279 discloses thermoplastic copolyester elastomers derived from terephthalic acid or 2,6-naphthalenedicarboxylic acid, an alkyl or alkenylsuccinic anhydride (side chain=7 to 30 carbon atoms) and 1,4 butanediol. U.S. Pat. No. 3,891,604 discloses segmented thermoplastic copolyetheresters derived from poly(tetramethylene ether)glycol, glycols containing 2 to 8 carbon atoms, alkyl or alkenyl succinic anhydrides and C.sub.8 to C.sub.16 aromatic dicarboxylic acids.